地滑/岩崩形成堰塞湖之運動模擬及參數探討

Abstract

堰塞湖是否發生取決於河道水文特性與坡面塊體下移行為，與水文、地質（岩性與構造）及地形條件有關。堰塞湖形成之潛感分析旨在預測可能發生災害所在，多透過已發生之案例進行統計分析，礙於堰塞湖相關資料常有不足，邊坡塊體之滑落過程又無法進行全尺寸之模型試驗，本研究嘗試以數值模擬方法為工具，探討影響大規模邊坡崩、滑後運移行為之重要因素，並檢討其堵塞河道進而形成堰塞湖之潛勢。本研究採用顆粒體離散元素法（Granular discrete element method）之三維模擬軟體（PFC3D），模擬邊坡崩滑體運移至河道之過程與堆積結果。藉由草嶺、九份二山及小林村三個堰塞湖案例進行模擬方法之驗證，接著針對詳細調查資料不足之合流坪堰塞湖進行案例重建。由模擬結果顯示，若適當控制微觀參數，則本研究之模擬方法可合理模擬大型邊坡崩、滑之運移過程。本研究進一步假設一系列虛擬組合條件以探討不同崩落塊體條件對於天然壩堆積形態之影響，考慮之條件包含滑落塊體之體積大小及落距高低，模擬結果顯示，當滑落塊體體積小於10萬立方公尺，則難以阻斷河道形成堰塞湖；低落差之滑落塊體沿河道方向堆積的壩寬較短，壩體在滲流作用下可能於溢流前即潰決之機會越大。本研究之模擬方法應能有助於進一步之堰塞湖形成潛勢研究。

The formation of a landslide dam depends on the conditions of the river hydrological characteristics and the run-out features of the slope mass; these conditions are related to hydrology, geology (including lithology and geologic structures) and morphology. Susceptibility analyses of landslide-dam formation aim to locate possible sites for landslide dams to occur; in general, these analyses made use of statistical method based on recorded landslide dam cases. However, the relevant data for statistics are often insufficient. It is neither possible to conduct full-size model test to explore the run-out process of unstable mass. This thesis attempts to investigate some important factors for the run-out behavior of landslide/avalanche masses by numerical simulation, and to examine the chance for a run-out mass to block the river valley so that forms a landslide dam. This study adopts the code of particle flow simulation PFC3D to model the run-out process of rock masses on slope along with a landslide or avalanche. Reconstructions by simulation for several real landslide-dam cases were conducted to verify the simulation approach. The validation demonstrates that the formation process of a large-scale landslide dam can be correctly modeled with appropriate assigned parameters. For a landslide-dam case without sufficient data or records, it is then possible to apply this approach to reconstruct what could have taken place. This study assumed a series of virtual conditions to explore a few variables that may determine whether a landslide-dam can be formed. The results indicate a few interesting trends. As the volume of the landslide is less than 100,000 m3, the final deposition tends not to block a river. Next, for a low elevation drop (from the landslide spot to the river base), the deposition after the run out of landslide tends to be more confined (not spreading out) in the river valley adjacent to the toe of the landslide slope. As a result, the dam width along the flow direction will be limited and prone to piping or slope failure very quickly. The simulation approach in the study should be applicable for further susceptibility study of landslide-dam formation.